As a transmission apparatus for use in vehicles such as a four-wheeled buggy vehicle, transmission apparatus of the type having a continuously variable transmission device by a belt power transmission mechanism have been used extensively. Such a non-stage transmission device of the belt type requires frequent maintenance, particularly in severe conditions. Therefore, in recent years such transmission apparatus have been replaced by hydro-static transmission (HST) devices.
For example, Japanese Patent Application Kokai No. H07-113454 discloses a hydro-mechanical transmission (HMT) apparatus, i.e., a combination of a mechanical transmission (MT) device and an HST device, for use in a motor bicycle. In this prior art transmission apparatus, an in-line piston pump and a piston motor which together constitute the HST device are disposed opposite in coaxial alignment, and cylinder barrels of the pump and the motor are made integral with an output shaft at the center so that the entire HMT apparatus is shaped like a cylinder and can be laid out, like a train of gears, etc. More specifically, the HMT apparatus is disposed at the rear of a single cylinder engine sideways-mounted on the vehicle body so that its output shaft runs parallel with a crank shaft, in other words the output shaft is so disposed as to extend in the vehicle width direction. Further, a primary speed-reduction mechanism and a secondary speed-reduction mechanism are disposed apart from each other on the right and left sides of the HMT apparatus, respectively, whereby the entire transmission apparatus is prevented from protruding horizontally beyond the engine.
Problems for Solution
However, for the case of the aforesaid prior art transmission apparatus, the in-line piston pump and the in-line piston motor which are relatively elongated in the axial direction are disposed opposite each other in coaxial alignment. This inevitably results in elongating, in totality, the piston pump and the piston motor in the axial direction (vehicle width direction). Because of this, particularly in some vehicles such as an ATV and a motor bicycle in which the transmission apparatus is disposed under a driver's seat of the ride-on type, there actually exists a strict limit on the layout of each component part of the transmission apparatus when trying to avoid interference with the driver's legs.
Further, because of the aforesaid arrangement that the pump and the motor are placed opposite to each other, the HST device section including the pump and the motor is inevitably located nearer to the inside of the transmission apparatus, therefore making it difficult to sufficiently cool the HST device whose heat generation is, by nature, greater than that of the MT device. As a result, troubles due to the rise in operating fluid temperature tend to take place. Besides, neither the maintenance nor the individual operation testing of the HST device section located nearer to the inside of the transmission apparatus is easy to carry out. Accordingly, the reliability of the HST device may drop and the durability/reliability of the entire transmission apparatus may be reduced.
Additionally, as described above, the HST device section is located nearer to the inside of the transmission apparatus, the operating fluid for the HST device must be commonized to either the engine oil or gear oil. However, generally the operating fluid of the HST device greatly differs in adequate temperature condition from the gear oil, let alone the engine oil. Accordingly, it is difficult to adequately maintain the characteristics of the HST operating fluid. Further, inconvenience, such as operating fluid deterioration and sludge contamination, tends to take place.
Bearing in mind the aforesaid drawbacks, the present invention was made. Accordingly, an object of the present invention is to reduce the axial dimension of the entire transmission apparatus while at the same time improving the degree of layout freedom and further to provide improved cooling and easy maintenance of the HST device section, by contriving the layout of the hydro-mechanical transmission (HMT) apparatus, particularly the layout of the HST apparatus section thereof.